1. Field of the Invention
The present invention relates to an evaluation technique in the field of the semiconductor fabrication, and more particularly, it relates to a technique for evaluating a crystal rate or crystallinity in steps of fabricating a silicon thin film.
2. Description of the Background Art
As to a thin film MOS transistor (hereinafter referred to as "TFT") which is obtained by converting an amorphous silicon thin film to a recrystallized film through solid-phase growth the, possibility of apply same to a liquid crystal display, memory or the like has recently been studied because of far superior electric characteristics as compared with a transistor comprising a polysilicon film formed by another method (T. Katoh, IEEE Trans. Electron Devices, Vol. ED-35 (1988), p. 923, for example).
Improved electric characteristics of such a TFT may be obtained by increasing the recrystallized crystal size. It has been recognized that this improvement strongly depends on the amorphous state of the amorphous silicon thin film before solid phase growth. That is, the amorphous silicon of which has characteristics that are varied with deposition conditions, conceivably contains pure amorphous phases and crystal phases in a mixed state. In order to increase the crystal grain size through solid phase growth, in particular, it is necessary to properly control the density of nuclei of crystal phases (hereinafter referred to as "crystal nuclei") contained in the amorphous silicon. If the density of such crystal nuclei is too large, for example, the number of crystal grains is increased since the crystal grains are grown from the nuclei of these crystal phases during the solid phase growth, to reduce the grain size. If the amorphous silicon is completely composed of pure amorphous phases alone with no crystal nuclei, on the other hand, crystal grains are hardly grown therein.
It is possible to control the density of the crystal nuclei by ion-implanting highly concentrated Si atoms etc. into an amorphous silicon thin film or a polysilicon thin film (T. Ohshima, T. Noguchi and H. Hayashi: Jpn. J. Appl. Phys. Vol. 25 (1986), L291).
If the crystal phases can be quantitatively evaluated in advance of solid phase growth, therefore, it is possible to effectively optimize process conditions so that the recrystallized crystal grain size is increased after solid phase growth. When such evaluation is carried out after recrystallization, it is possible to decide whether the recrystallized thin film is defective or not.
To this end, there have generally been proposed various evaluation methods such as Raman spectrometry, X-ray diffraction, Rutherford back scattering and transmission electron beam diffraction.
As to evaluation of amorphous silicon by the Raman spectrometry, Raman band strength of crystals is regarded as the measure for transition from amorphous phases to crystal phases. On the other hand in Raman spectroscopy, a Raman spectrum is separated into crystal phase components and amorphous phase components, to estimate the volume ratio of the crystal phases from integrated intensity values thereof. In "Raman-Scattering Studies of Silicon-Implanted Galluim Arsenide" by M. Holtz and R. Zallen, J. Appl. Phys. Vol. 59 (1986), p. 1946, it is assumed that peak intensity of scattered light obtained by Raman spectrometry correlates with crystallinity in consideration of a single substance, and the scattered peak intensity is reduced as crystallinity is deteriorated. Unfortunately, Raman spectrometry, measuring steps are complicated and a long time is required for measurement, while the measuring apparatus is increased in size.
Evaluation of crystallinity by the X-ray diffraction has problems similar to the above.
Rutherford back scattering is adapted to accelerate light atoms of ionized hydrogen or helium and introduce the same into a sample, to evaluate crystallinity of the sample by analyzing the energy of these ions which are scattered by atomic nuclei and jump out from the sample. As to evaluation of crystallinity, the area of a surface peak of an align spectrum is related to the density of atoms which are displaced from lattice positions, i.e., defective density, to quantify the crystallinity, as in "Back Scattering Spectrometry" by W. K. Chu, J. W. Mayer and M. A. Nicolet, Academic Press, New York, Chap. 2 (1978), In such Rutherford back scattering, however, a long time is required for measurement. On the other hand, transmission electron beam diffraction is adapted to introduce a highly accelerated electron beam into a sample to obtain the crystal grain size from an image of the transmitted electron beam, and hence the sample must be worked into an extremely small thickness, to be capable of transmitting the electron beam. Such working requires a long time, and a longer measuring time is required for transmission election beam diffraction as compared with the Rutherford back scattering. Further, such measurement cannot be carried out during fabrication steps, since this is a destructive test.
In addition, all these prior art methods require complicated and large-sized measuring apparatus, which must be operated by skilled personnel.